mcrypt is intended to be a replacement of the old unix crypt(1)
under the GNU General Public License. Unix crypt(1) was a popular
file encryption program in unix boxes.

It was based on the Enigma encryption algorithm but it was considerable
trivialized. Since this was not adequate, even for individual privacy
needs, mcrypt was created as a similar program using some modern
block encryption algorithms.

Mcrypt also has a compatibility mode with unix crypt(1) and with
Solaris des(1). It supports all the algorithms and modes found in
libmcrypt and it is very extendable.

At the time writing this, it supports the algorithms: BLOWFISH,
TWOFISH, DES, TripleDES, 3-WAY, SAFER, LOKI97, GOST, RC2, RC6, MARS,
IDEA, RIJNDAEL, SERPENT, CAST, ARCFOUR and WAKE.

Block algorithms are implemented in modes: CFB, CBC, ECB, OFB (8
bit and n bit, where n is the size of the algorithm's block length).
For a brief description of the algorithms and the modes look at the
mcrypt manpage (this may be out of date).  In mcrypt it is on the
user to decide which algorithm he considers best for encrypting his
data.
